Chapter 2. Prehension

xd
Chapter 5 - Movement Before Contact 139
internal model of
inverse-dynamics
cembiu-ce&ellum
and Pam led nucleus +u
desired U* somaiwensoq feedback
trajectory
U
spin~beuum * and Magno d nudeus *
internal model of
dynamics
X
movement
pa ttem
Figure 5.13. Hierarchical feedback-error learning model for control
and for learning of voluntary movement. Somatosensory feedback
x from the periphery is initially used by the motor cortex for
adjusting the on-going motor command u. As the system learns
the internal model of its dynamics, the transcortical corrections
are replaced by a faster cerebellar feedback pathway that can more
quickly adjust the motor command (using x*). Eventually, the
system develops an internal model of the inverse dynamics, and
can use an even faster, feedforward cerebellar pathway (generating
u* to replace u) that can anticipate changes in the periphery (from
Kawato, Furukawa & Suzuki, 1987; adapted by permission).
the trajectory. Because the input is the desired motor pattern, xd, the
output is a feedforward motor command. This motor command, in the
form of muscle torques, is sent to the musculo-skeletal system which
in turn produces some actual motor pattern, x. The results of the
movement are received back at the motor cortex on the line labelled
'somatosensory feedback' (through the medial lemniscus system) and
feedback control reduces the error in Xd-X. However, delays in this
path can limit the usefulness of this control method.
Kawato et al. 's hierarchical feedback-error learning control model
suggests that two cerebellar regions can participate in feedforward and
feedback control using internal models of the limb's dynamics.
Besides the feedback path described in the previous paragraph, the re-
sults of the movement are received in the spinocerebellum, a part of
the cerebellum that has two somatotopically organized maps of the en-